package org.apache.lucene.util;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.lucene.index.IndexReader;

/**
 * Common util methods for dealing with {@link IndexReader}s.
 *
 * @lucene.internal
 */
public final class ReaderUtil {

    private ReaderUtil() {
    } // no instance

    /**
     * Gathers sub-readers from reader into a List.
     * 
     * @param allSubReaders
     * @param reader
     */
    public static void gatherSubReaders(List<IndexReader> allSubReaders, IndexReader reader) {
        IndexReader[] subReaders = reader.getSequentialSubReaders();
        if (subReaders == null) {
            // Add the reader itself, and do not recurse
            allSubReaders.add(reader);
        } else {
            for (int i = 0; i < subReaders.length; i++) {
                gatherSubReaders(allSubReaders, subReaders[i]);
            }
        }
    }

    /** Recursively visits all sub-readers of a reader.  You
     *  should subclass this and override the add method to
     *  gather what you need.
     *
     * @lucene.experimental */
    public static abstract class Gather {
        private final IndexReader topReader;

        public Gather(IndexReader r) {
            topReader = r;
        }

        public int run() throws IOException {
            return run(0, topReader);
        }

        public int run(int docBase) throws IOException {
            return run(docBase, topReader);
        }

        private int run(int base, IndexReader reader) throws IOException {
            IndexReader[] subReaders = reader.getSequentialSubReaders();
            if (subReaders == null) {
                // atomic reader
                add(base, reader);
                base += reader.maxDoc();
            } else {
                // composite reader
                for (int i = 0; i < subReaders.length; i++) {
                    base = run(base, subReaders[i]);
                }
            }

            return base;
        }

        protected abstract void add(int base, IndexReader r) throws IOException;
    }

    /**
     * Returns sub IndexReader that contains the given document id.
     *    
     * @param doc id of document
     * @param reader parent reader
     * @return sub reader of parent which contains the specified doc id
     */
    public static IndexReader subReader(int doc, IndexReader reader) {
        List<IndexReader> subReadersList = new ArrayList<IndexReader>();
        ReaderUtil.gatherSubReaders(subReadersList, reader);
        IndexReader[] subReaders = subReadersList.toArray(new IndexReader[subReadersList.size()]);
        int[] docStarts = new int[subReaders.length];
        int maxDoc = 0;
        for (int i = 0; i < subReaders.length; i++) {
            docStarts[i] = maxDoc;
            maxDoc += subReaders[i].maxDoc();
        }
        return subReaders[ReaderUtil.subIndex(doc, docStarts)];
    }

    /**
     * Returns sub-reader subIndex from reader.
     * 
     * @param reader parent reader
     * @param subIndex index of desired sub reader
     * @return the subreader at subIndex
     */
    public static IndexReader subReader(IndexReader reader, int subIndex) {
        List<IndexReader> subReadersList = new ArrayList<IndexReader>();
        ReaderUtil.gatherSubReaders(subReadersList, reader);
        IndexReader[] subReaders = subReadersList.toArray(new IndexReader[subReadersList.size()]);
        return subReaders[subIndex];
    }

    /**
     * Returns index of the searcher/reader for document <code>n</code> in the
     * array used to construct this searcher/reader.
     */
    public static int subIndex(int n, int[] docStarts) { // find
        // searcher/reader for doc n:
        int size = docStarts.length;
        int lo = 0; // search starts array
        int hi = size - 1; // for first element less than n, return its index
        while (hi >= lo) {
            int mid = (lo + hi) >>> 1;
            int midValue = docStarts[mid];
            if (n < midValue)
                hi = mid - 1;
            else if (n > midValue)
                lo = mid + 1;
            else { // found a match
                while (mid + 1 < size && docStarts[mid + 1] == midValue) {
                    mid++; // scan to last match
                }
                return mid;
            }
        }
        return hi;
    }
}
